A solar cell is formed of two (or more) semiconductor layers in facing contact with each other at a semiconductor junction. When illuminated by the sun or otherwise, the solar cell produces a voltage between the semiconductor layers. Advanced solar cells may include more than two semiconductor layers and their respective pairwise semiconductor junctions. The various pairs of semiconductor layers of the advanced solar cells are tuned to the various spectral components of the sun to maximize the power output of the solar cell.
The voltage and current output of the solar cell are limited by the materials of construction and the surface area of the solar cell. Most commonly, a number of solar cells are electrically interconnected in series and/or parallel arrays to form a solar cell structure that produces higher voltages and/or higher currents than are possible with the single solar cell. Such solar cell structures are now used in both space and terrestrial applications.
The solar cell structure works well when all of the solar cells are illuminated with about the same illumination intensity. However, if one of the solar cells of the solar cell structure is shaded while the others remain fully illuminated, the shaded solar cell is subjected to a reverse-bias condition by the continuing voltage and current output of the remaining solar cells.
Fortunately, each solar cell may be protected against the damage arising during the reverse-bias condition by a parallel diode that blocks current when the solar cell is not reverse biased, but passes the impressed current when the solar cell is reverse biased. The diode thus protects the individual cell against reverse-bias damage.
A number of diode configurations are in use and are operable, but each has its drawbacks. In one approach, the discrete by-pass diodes are placed to one side of the solar cells, necessitating the use of wiring that extends between the solar cell and the by-pass diodes. In another configuration, the discrete by-pass diode is bonded to the backside of the solar cell and interconnected to the semiconductor layers of the solar cell with leads. This approach potentially exposes the solar cell to stresses that may cause it to crack if pressure is applied against the by-pass diode during assembly bonding. In a variation, the by-pass diode is placed into a recess on the back side of the solar cell, but this approach is operable only for relatively thick solar cells. In another configuration, the diode is grown onto the front surface of the solar cell as part of the deposition process and then interconnected to the next cell in series. This approach is complex and causes assembly difficulties as well as reduced production yields and reduced solar cell efficiency. In yet another configuration, the diode is also grown into the front surface of the solar cell and interconnected with discrete or lithographic techniques. This approach is also complex, and has reduced production yields, and reduced solar cell efficiency.
Another problem experienced with solar cell structures is heat removal. For all solar cells, but particularly for concentrator solar cells, heat produced in the solar cell must be removed through the back side of the solar cell so that the solar cell does not exceed its preferred operating temperature for optimal performance. The presence of the by-pass diode must not interfere with the heat removal, and desirably the by-pass diode structure facilitates heat removal from the solar cell.
There is a need for an improved approach to the protection of solar cells against reverse-bias damage. Additionally, there is a need for an improved approach to removing heat from solar cells for improved performance and reliability. The present invention fulfills these needs, and further provides related advantages.